In optical communications, signal light is transmitted via an optical fiber. Since power of the signal light drops when the signal light travels along the optical fiber by a long distance, an optical fiber amplifier is used to amplify the power. More specifically, the optical fiber amplifier amplifies signal light power by the use of the stimulated emission process using excited state rare-earth ions by inputting pump light into an optical fiber doped a rare-earth element (amplification medium).
Currently, optical fiber amplifiers having the amplification medium doped with erbium are gaining popularity. Such an amplification medium and such an optical fiber amplifier are respectively referred to as EDF (Erbium Doped Fiber) and EDFA (Erbium Doped Fiber Amplifier).
Japanese Laid-open Patent Publication No. 08-248455 discloses an optical fiber amplifier in which two stages of EDF are arranged with a variable optical attenuator inserted therebetween. Such a two-staged EDF optical fiber amplifier design is a widely used structure.
The above-described two-staged EDF of optical fiber amplifier performs an efficient amplification operation by receiving different pump lights at respective stages.
More specifically, a first-stage EDF (at a signal input side) receives an pump light falling within a 0.98 μm band that is excellent in noise characteristics but low in amplification efficiency, and a second-stage EDF (at a signal output side) receives an pump light falling within a 1.48 μm band that has a high amplification efficiency.
Such pump light are generated by a pump light source such as an LD (Laser Diode). Such a pump light source continuously outputs a pump light at a substantially high power within a range of 100 mW—several hundreds mW in response to a current flowing therethrough. The pump light source fatigues more quickly than other optical components. Output power of the pump light source drops gradually.
The EDFA largely changes in characteristics as an optical amplifier depending on the output power of the pump light incident on the EDF. When the output power drops below a lower limit thereof, replacing steps are preferably taken, for example, by replacing the light excitation source with a new one, or by replacing one component. For example, the output power of the pump light may be initially 300 mW with a current of certain value flowing through the pump light source. If the output power starts to drop later on and only an output power lower than 150 mW is obtained even with the same current flowing (also in the case of a trouble in which the output power suddenly drops to zero mW), it is determined that the pump light source suffers from a performance deterioration.
If the output power of the pump light within the 1.48 μm band directed to the second EDF drops in the two-staged EDFA, the output level of the signal light also drops. Therefore, the output level of the second stage signal light is monitored, and when the output level drops below a predetermined lower limit, it is determined that the pump light source suffers from a performance deterioration.
In contrast, if a drop takes place in the output power of the pump light within the 0.98 μm band directed to the first EDF, noise characteristics as an optical amplifier are deteriorated, but the output level of the signal light output from the first stage is not so largely affected. For this reason, even if the output level of the signal light from the first stage is monitored, it is difficult to determine whether the pump light source is degraded.
Available as another determination method is the use of back power monitor provided by the LD. However, currently, particularly, the accuracy of the back monitor of the LD with an FBG (Fiber Bragg Grating) is low, and unstable depending on a polarized wave state of the pump light, etc. Because of this, there are times when the amplifier is determined to be degraded even though the LD is not actually degraded.
In another method, branching part of the pump light, such as a beam splitter, is arranged immediately subsequent to the output of the LD, and the output power of the branched pump light is directly monitored. With this method, the deterioration determination is performed. However, costs for mounting the branching means are required, and the LD needs to output extra power for branching the pump light. Costs for installing such an LD are additionally needed.